The present invention is directed to improving the efficiency of color correction operations. In particular, this invention capitalizes on the need to store digitized images in a compressed format, and incorporates color correction with compression or decompression operations so as to accomplish certain aspects of color correction on compressed images in a computationally efficient manner.
Data compression is required in data handling processes, where too much data is present for practical applications using the data. Commonly, compression is used in communication links, to reduce the transmission time or required bandwidth. Similarly, compression is preferred in image storage systems, including digital printers and copiers, where "pages" of a document to be printed are stored temporarily in precollation memory. The amount of media space on which the image data is stored can be substantially reduced with compression. Generally speaking, scanned images, i.e., electronic representations of hard copy documents, are often large, and thus make desirable candidates for compression.
The present invention has application to both lossy and lossless compression schemes. The DCT (Discrete Cosine Transform), a variable-length encoding standard, disseminated by the JPEG committee, is a lossy compression scheme which reduces data redundancies based on pixel to pixel correlations. The Lempel-Ziv scheme is for lossless compression, including for example, the LZW scheme described in the U.S. Pat. No. 4,558,302 and U.S. Pat. No. 4,814,746 patents. Like LZW, the Huffman scheme and other lossless compression equivalents are capable of compressing data without any loss of image data, albeit typically at a compression ratio that is lower than that for lossy compression. Generally, an image does not change very much on a pixel to pixel basis and therefore has what is known as "natural spatial correlation". In natural scenes, correlation is generalized, but not exact. Noise makes each pixel somewhat different from its neighbors, but the natural spatial correlation enables not only the compression of digital image data, but the performance of certain image processing operations on the compressed data. Contrariwise, in synthetic graphics and computer-generated text, the correlation tends to be much higher, with colors limited to a small set. The limited set of colors and high correlation make these image types good candidates for lossless compression, at possibly higher compression ratios than those achievable with lossy compression schemes.
Heretofore, a number of patents and publications have disclosed image compression and color correction, the relevant portions of some may be briefly summarized as follows:
U.S. Pat. No. 5,408,425 to H. S. Hou, issued Apr. 18, 1995, hereby incorporated by reference for its teachings, discloses a processor and method of computation for performing a discrete cosine transform.
U.S. Pat. No. 5,31 9,724 to S. M. Blonstein et al., issued Jun. 7, 1994, teaches an apparatus and corresponding method for compressing still images while remaining compatible with a JPEG transformation.
U.S. Pat. No. 4,275,413 to Sakamoto et al. discloses a color space transformation where information is placed into lookup tables and stored in a memory--where the lookup table relates input color space to output color space. Sakamoto teaches a "unit cube interpolation unit" having known vertices. The lookup table is commonly a three dimensional table since color is typically defined with three variables.
U.S. Pat. No. 5,581,376 to Harrington teaches the conversion of input device signals Rs, Gs, Bs, generated by an image input terminal, to colorimetric values Rc, Gc, Bc, the colorimetric values being processed to generate address entries into a lookup table to convert them to Cx,Mx,Yx,Kx colorant signals or any multi-dimensional output color space, which includes but is not limited to CMYK or spectral data. Values not directly mapped may be determined using tetrahedral interpolation over a hexagonal lattice where the lattice is formed by offsetting every other row in at least one dimension.
In accordance with the present invention, there is provided a method for performing color correction of a compressed digital image, comprising the steps of: retrieving from memory a compressed unit of image data; partially decompressing the compressed unit to produce a partially decompressed unit representing part of the information describing a region of the image in a first color space; performing color correction on the partially decompressed unit to produce a corrected unit in the first color space; subsequently decompressing the entire corrected unit to produce decompressed, corrected image data; and converting the decompressed, corrected image data into a second color space image for rendering.
One aspect of the invention deals with a basic problem in digital image processing systems--the memory and computationally intensive operations of image processing, particularly color correction. This aspect is further based on the discovery of a technique that alleviates this problem. The technique accomplishes the color conversion of an image segment or block while it is in a digitally compressed form--produced by lossy or lossless compression operations.
This technique can be implemented, for example, by the modification of basic JPEG or LZW compression engines, where correction operations are performed on the compressed image data (compressed units). Accordingly, the technique has application to any number of systems, including digital printers and copiers where there is a requirement to provide color correction of the digital image. A machine implementing the invention can include a data or image processing system having the capability of image compression. The techniques and system described herein are advantageous because they are efficient and result in the ability to accomplish basic image processing with little additional hardware or processing as compared to other memory intensive approaches.